A strong central magnetic field and a wide imaging space are desired for a medical nuclear magnetic resonance system. The strong central magnetic field may improve an imaging resolution. As compared with a cylindrical configuration, an open type magnet system formed by spaced super-conducting coils may provide patients with a magnetic field perpendicular to a sampling space, thereby having an imaging efficiency of √{square root over (2)} times higher than that of a parallel magnetic field. Thus, the perpendicular field has a higher imaging efficiency than the parallel field. On a main magnetic field is superposed a gradient magnetic field which changes over time which generates spatial codes of the imaging space. Further by means of stimulating of the sample realized by radio-frequency coils, clear diagnosing images can be provided for patients. Additionally, the magnet system composed of spaced super-conducting coils may provide a stronger magnetic field and a wider imaging area than a permanent magnet, as well as a space allowed for an on-line surgical treatment, and meanwhile the patients will not suffer from claustrophobia.
Presently, the medical nuclear magnetic resonance magnet system usually uses neodymium-iron-boron (NdFeB) permanent magnets, resistance magnets and superconducting magnets, etc. to generate the magnetic field. The magnetic field generated by common permanent magnets has an intensity of 0.5 T or below which is greatly influenced by environment temperatures. To stabilize the center magnetic field, a temperature controlling system is often used to ensure that the magnets have constant temperatures. The resistance magnes can provide a relatively high magnetic field, but has a considerable power consumptions, and moreover the magnetic field thereof has a poor stability which is influenced by a ripple factor of a power supply. With a development of new superconducting materials and cryogenic technology, superconducting magnets can be operated continuously for several years or more, through using non-volatile liquid helium immerging cooling.
A new type of magnet system is developed by replacing the permanent magnet system with a superconducting magnet system and using iron yokes for correcting the magnetic field, shielding the magnetic field, and providing a magnetic circuit. The resulting magnet system is lightweight and is relatively compact in structure. An electromagnetic force of up to dozens of tons may be generated between upper/lower iron yokes and the symmetrical distributing superconducting coils due to interactions therebetween, which tends to cause the coils to be instable. Thus, a strong external supporting means is required in order to eliminate the instability of coils. The strong external supporting means tends to deform at a cryogenic condition, which in turn causes deformations of the coils, even breakage of the coil structures, thereby causing “a quench phenomenon”. On the other hand, the external supporting means is large in volume and is complex in structure, such that the whole magnet system is complex in structure. Finally, heat will flow from the external supporting means into the cryogenic system, such that liquid helium consumption is relatively high.